Page 176 - DCAP406_DCAP_207_Computer Networks

Unit 11: Transport Layer

transport entity suspends transparently within to_net until there is room in the window. Apart Notes
from this transparent suspension mechanism, some explicit procedures called by the transport
entity to block/unblock itself are given above:
sleep() – This procedure is called when the transport entity logically needs to wait for an
external event to happen. After calling the sleep procedure, the (main stream of the) transport
entity is blocked.
wakeup() – This procedure is called by the event handling procedure (i.e., packet_arrival() )
– To unblock the sleeping (main stream of the) transport entity.
User programs call most of procedures in the transport entity directly. However, there are two
procedures that are effectively (software) interrupt routines and are called only when the main
stream of the transport entity is sleeping. They are given as below:
packet_arrival() – This is triggered by the packet arrival event. The underlying network
layer creates this procedure.

clock() – The clock ticking event triggers this procedure.
A flow control mechanism based on credit is used in the example transport entity:
When an application calls RECEIVE, a special credit message is sent to the transport entity
on the source machine and is recorded in the conn array.

When SEND is called, the transport entity checks to see if a credit has received on the
specified connection.
If so, the message is transmitted in multiple packets, if needed, and the credit
decremented;
If not, the transport entity changes itself to sleep until a credit receives.
In the transport entity, each connection is expresses in one of the following seven states:
Idle – Connection not established yet.

Waiting – CONNECT has been executed, CALL REQUEST sent.
Queued – A CALL REQUEST has arrived; no LISTEN yet.
Established – The connection has been established.
Sending – The user is waiting for permission to send a packet.
Receiving – A RECEIVE has been done.

Disconnecting – A Disconnect has been done locally.
11.3.3 The Example as a Finite State Machine

Figure 11.1 shows an example of finite state machine. In the finite state machine, each entry has
an optional predicate, an optional action and the new state.

Task What is the difference between flow control and congestion control?
The key protocols of the Transport Layer are Transmission Control Protocol (TCP) and User
Datagram Protocol (UDP). TCP enables reliable data delivery service with end-to-end error
detection and correction. UDP facilitates low-overhead, connectionless datagram delivery service.
Both protocols are responsible for delivering data between the session layer and the network
layer.

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